1. Field of the Invention
The invention relates to the stimulation of petroleum wells in general and to the stimulation of oil wells in particular.
2. Prior Art
Petroleum wells typically comprise a well bore bordered by a casing. One or more production lines are positioned within the casing so that the petroleum may be carried to the surface. In the petroleum bearing portion of the formation, the casing will be perforated to allow the oil to flow into the well and be pumped up the production line. These perforations are typically done by piercing the casing with specially configured bullets, small charges, highly compressed air or other fluids, or a variety of other well known prior art methods. When the casing is initially punctured, there will typically be several perforations made to facilitate the flow of oil into the well at various points in the formation.
Unfortunately, many oil bearing strata also contain water. Of course, the petroleum will float atop the water. Thus, the upper perforations will allow oil to flow into the well while the lower perforations will often allow water to flow into the well, all of which must then be brought to the surface, where the oil can be separated from the water.
Raising the water is obviously undesirable. First, there is an expense associated with running the pump to lift the water. Every gallon of water raised represents a cost that cannot be recouped. More importantly each gallon of water raised also represents a gallon of oil that was not produced on that day. Wells that produce a high volume of water per day relative the amount of oil produced are obviously not very efficient.
The amount of water produced by a well can change over time. One way this is believed to occur is through “water coning.” As the oil and water flow through the formation, they will wet the sands in the vicinity of the well. As the water flows through sands, it will climb the outside of the casing, displacing the oil. Thus, over time, the formation may change from a configuration similar to the one shown in FIG. 1A to one more similar to the configuration shown in FIG. 1B.
As the water rises around the well casing, it will cover the perforations that previously allowed petroleum to flow into the well. Water and oil are generally immiscible, so the water will effectively prevent petroleum from flowing through the covered perforations. This will reduce the number of perforations through which the petroleum can flow, which will reduce the barrels of oil produced by the well per day (bopd) and simultaneously increase the barrels of water produced by the well per day (bwpd). Thus, water coning can reduce the petroleum output of a well and simultaneously increase the cost of production, as more water must be lifted for each barrel of petroleum.
A second problem for many petroleum wells, particularly those producing heavy crudes, i.e. those whose average API gravity is below about seventeen degrees, is viscosity. Such heavy crudes can be difficult to pump because of their high viscosities and can cause pressure in the production line to rise, making pumping more difficult. This can damage and cause premature wearing in well equipment, increasing the cost of production. Such viscosity problems are commonly addressed by pumping a diluent—any lower API gravity oil—down hole so that it can mix with the oil in the well. This is obviously inefficient, since what is pumped down must be pumped back up. Therefore, the use of diluents is preferably minimized. Accordingly, a well stimulant meeting the following objectives is desired.